1. Field of the Invention
This invention relates generally to a method and apparatus for measuring an electrical charge on a surface, and more particularly to a system of one or more micro-cantilevers for measuring charges on a surface layer on a substrate during semiconductor processing on semiconductor wafers.
2. Related Art
Plasmas are widely used in the semiconductor industry for processing to delineate fine line pattern and deposition at low temperature, e.g. plasma etching. During plasma exposure the wafer is exposed to a bombardment of ions, electron, photons, and x-rays which can lead to a charge build-up on the semiconductor device. Such a charge build-up, known as wafer surface charging, can degrade or destroy the device. If such charges develop, a voltage can develop which could cause irreversible damage to the device, especially metal oxide semi conductor (MOS) gate dielectrics. In recent years, it has been reported that plasma non-uniformity across the wafer is the predominant cause of charging. This non-uniformity can arise from non-uniformities in RF current flow, electron current flow, and ion current flow. Such charging can also arise from handling and/or cleaning a semiconductor device. These problems are becoming more important as gate oxides become thinner, and hence, more vulnerable to surface charging.
It would be beneficial to measure in situ, in a plasma reactor, the charge on a wafer to determine the magnitude of such charges occurring during the manufacturing process. This could lead to improved manufacturing processes. Additionally, if one could measure the charge on a wafer that occurred during the manufacturing process, one may be able to use such measurement as a quality control means, i.e. by discarding chips have too great a charge and therefore, a high likelihood of damage.
Previous efforts in this area have not yielded a suitable method and apparatus for measuring a charge on a wafer during the manufacturing process. One way that has been used in the past to obtain measurements of charge is to attach leads directly to a specimen. Another way of determining charge is to measure degradation of the transistor after processing. Such previous efforts include:
Murakawa, et al., "Mechanism of Surface Charging Effects on etching Profile Defects," Jpn. J. Appl. Phys., Vol 33 (1994) discloses using a magnet placed under a grounded electrode to create a magnetic field at the wafer center. A probe measures the plasma potential along the wafer. The probe comprises a silicon wafer with an A1 pad with contacts leading to Cu wires extending out of the chamber to RF chokes and low-pass filters and then to a DC voltmeter. This is not an elegant solution as it requires fishing wires in and out of the reactor chamber.
Another commercially available method of measuring charge comprises a detection device which can be employed outside the reactor chamber after processing of a device is complete. However, this solution is not entirely satisfactory because it measures an accumulated affect over the total processing time and cannot provide specific information about charge development during processing.
None of these previous efforts teach or suggest all of the elements of the present invention, nor do any disclose the benefits and utility of the invention.